A series of experiments are described relating to the effect of aging on the function of the B-cell population. Cell transfer techniques using congenic donor-recipient pairs are proposed. We will attempt to determine if the B-cell population of old mice differs from that of young animals in its ability (a) to establish and maintain itself in the cell transfer recipient, and (b) to respond to TI and TD antigens. The effect of age on the duration of B-cell memory and on the "regenerative" capacity of the B-cell population will also be studied in this cell transfer model. In all transfer studies the use of cell surface markers and Ig allotypic markers will permit confirmation that the donor cells give rise to the PFC and synthesize the antibody. In addition, we propose to determine if there is an age related change in the IgG subclass distribution of the antibody produced or whether there is an age-related loss of a B-cell sub-population. It should be emphasized that cell transfer methods, while frequently employed in immunologic studies, have an inherent uncertainty as to which cell type (donor or recipient) is actually mounting the immune response. The assumption that when a lethally irradiated host is employed it is always the donor cells which respond, has recently been challenged. The novel approach adopted in the present proposal is the use of congenic donor-recipient combinations so that the cell type generating the response can be unequivocally identified.